This invention relates generally to steam turbines and more generally to methods and apparatus for retaining material in pockets formed in hybrid buckets and to reducing stresses induced to the buckets.
Turbine buckets (blades) operate in an environment in which they are subject to high centrifugal loads, vibratory stresses, and a varying angle of flow incidence to the bucket. Vibratory stresses are increased when these loads and stresses approach bucket natural resonant frequencies. The magnitude of the vibratory stresses when a bucket vibrates in resonance is proportional to the amount of damping present in the system (wherein damping includes material, aerodynamic and mechanical components) and the stimulus level. For continuously coupled buckets, the frequency of vibration is a function of the entire system of blades, and not necessarily that of individual blades.
A hybrid bucket comprises a turbine bucket (for example, a steam turbine bucket or a gas turbine bucket) made primarily of a metallic substance but with at least one “pocket” of a non-metallic composite filler material. The filler material may comprise a polyimide or another type of polymeric resin (or combinations thereof) with continuous glass, carbon, KEVLAR® or other fiber reinforcement to achieve a composite matrix with the original airfoil surface. Composite matrix are now being designed to be used in units that have high bucket temperatures during windage conditions (low flow, high speed “wind milling” of buckets). However, very stiff high temperature composites do not adhere well to metal.
Many issued patents have been directed to turbine blades fabricated from composite materials. For example, U.S. Pat. No. 5,720,597, entitled “Multi-Component Blade for Gas Turbine,” describes gas turbine aircraft blades constructed of metal and foam are provided with a composite skin, an erosion coating, or both, and U.S. Pat. No. 6,139,728, entitled “Poly-Component Blade for a Steam Turbine,” discloses configurations similar to those disclosed in U.S. Pat. No. 5,720,597, but for steam turbines. However, in both of these applications, the size, shape, and location of a pocket to be formed in the bucket is limited. In addition, U.S. Pat. No. 6,042,338, entitled “Detuned Fan Blade Apparatus and Method,” describes a “propulsion engine fan” and various types of blades with different pocket locations, but does not disclose blades of essentially one pocket with different rib structures. Further, the disclosure is limited to pockets with radial location from a tip to 5%-38% span and chord wise from 15% to 35% from the leading edge and 20% to 45% from the trailing edge with similar limitations on the second or alternative pocket design. Moreover, none of these patents, describe or suggest the creation of through windows in a pocket for the purpose of mechanical assistance in holding polymer or composite into a bucket.